HOUSTON (Reuters) - Forecasters predicted heavy rains and flooding for the Houston area as Tropical Storm Humberto formed on Wednesday in the northern U.S. Gulf.

Hurricane trackers were also keeping an eye on a second storm system making its way west in the central Atlantic after being declared Tropical Depression 8 on Wednesday.

The U.S. National Hurricane Center said that a system offshore of Galveston, Texas, had become Tropical Storm Humberto.

At 1 p.m. CDT (2 p.m. EDT), Humberto was located 70 miles

south-southwest of Galveston and moving north at 6 mph (9 kph) with maximum sustained winds of 45 mph (75 kph).

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"Additional strengthening is possible prior to landfall," a hurricane center advisory said. "Rainfall amounts of 5 to 10 inches are expected along the middle and upper Texas Coast and in extreme southwestern Louisiana."

Humberto already was affecting the Houston Ship Channel, which serves one of the United States' busiest ports and primary oil-refining centers.

Pilots, who guide ships through the 50-mile (80-km) waterway, halted boarding ships at the channel entrance at midday on Wednesday due to rough seas.

The U.S. Coast Guard said pilots planned not to resume guiding ships up the channel until after Humberto passes.